1. Field of the Invention
This invention is generally related to the minimization of corrosion of titanium and titanium alloy metal surfaces and, more specifically, to the provision on at least a portion of the metal surface of rhodium metal.
2. Description of the Prior Art
Titanium and titanium alloys are, largely due to their generally high corrosion-resistance properties, widely used in industry as construction material or linings for vessels, piping and the like.
However, unacceptably high rates of corrosion of titanium can occur at elevated temperatures when in contact with strong acid media, e.g., aqueous media containing any of the strong mineral acids, such as nitric acid, phosphoric acid, sulfuric acid, hydrohalic acids (e.g., HBr, HCl, HI and HF), and the like, or any of the strong carboxylic acids, such as oxalic acid, formic acid, acetic acid and the like. Also, aqueous media containing dissolved salts of some of the above acids can vigorously attack titanium and titanium alloys.
Various compounds have been proposed for use as anticorrosive agents for titanium. Thus, U.S. Pat. No. 3,457,103 suggests use of siliceous compounds in the offending corrosive media. Also Fe.sup.3+, Cu.sup.2+ and Pt.sup.4+, as well as ions of Au, Hg, Zn, Co, Al and Mg, have been found to decrease rates of corrosion or to passivate titanium in certain media. See I. Ya. Klinov, Corrosion and Protection of Materials Used in Industrial Equipment pp. 79-90 (Consultants Bureau 1962); Corrosion vol. 19, No. 6, pp. 217t-221t (1963); N. G. Feige et al., Chem. Enq. Prog., vol. 66, No. 10, pp. 53-56 (1970); J. B. Cotton, Chem. Enq. Prog., vol. 66, No. 10, pp. 57-62 (1970); T. Koizumi et al., Corrosion and Corrosion Control, pp. 318-323 (J. Wiley & Sons 1973); Titanium Science and Technology, vo. 4, pp. 2383-2393 (1973); L. C. Covington, Titanium Science and Technology, vol. 4, pp. 2395-2403 (1973).
Oxyanions (SO.sub.4.sup.2-, NO.sub.3.sup.1-, CrO.sub.4.sup.2-, PO.sub.4.sup.3- and CO.sub.3.sup.2-) have been found to inhibit pitting of titanium in certain systems containing halide ions. See, e.g., T. Koisumi, et al., supra at pp. 2388-2392. Also, NaBr has been found to inhibit titanium corrosion in fuming nitric acid. See I. Ya. Klinov, supra at p. 87. However, the addition of such chemical agents of the offending corrosive liquid modifies process stream compositions and can lead to processing difficulties.
Alloying methods have been developed which provide corrosion protection without the disadvantages of such anticorrosion agents. However, alloying is time-consuming and expensive, and is impracticable for protection of existing chemical process apparatus. Moreover, Ti alloys are themselves not completely resistant to corrosion attack by strong acid media, as is acknowledged in U.S. Pat. No. 3,457,103.
Thus, for example, M. Stern, et al., J. Electrochem. Soc., vol. 19, No. 9, 755-759 and 759-764 (1959) observed corrosion of alloys of Ti and Pd, Pt, Pd, Rh, Ru, Ir, Os, Re or other metals in contact with boiling 10% HCl. Stern et al. also noted that the Ti in Ti-noble metal alloys corrodes at an accelerated rate when first placed in contact with a corrosive liquid and that the Ti continues to corrode until the atomic ratio of noble metal to Ti on the surface increases.
See also Kolyada et al., 88 Chem. Abs. 93630k (1978) (Ti alloys containing Rh and Y). Keinina, et al., 84 Chem. Abs. 8149y (1976) used Rh--Ti alloys as electrodes in the electroreduction of organic compounds. Eremenko, et al., 79 Chem. Abs. 10422j (1973) studied phase diagrams of Ti--Rh alloys.
Ti has been suggested as a suitable material of construction in manufacture of boiling nitric acid in contact with Pt--Rh alloy catalyst. Roy et al., 89 Chem. Abs. 9380p (1978). S. Z. Kostic et al., Br. Corros. J., vol. 9, No. 4, 211-215 (1974), in earlier work, also found tested Ti alloys to be passive in HNO.sub.3 solutions in contact with Pt--Rh alloy catalyst, although these researchers noted that while the Ti in such a catalytic system remained in the passive region, the corrosion potential of the Ti shifted toward more positive values, i.e., moved toward a less passive state and, hence, closer to an active corrosion region.
In view of the various problems presented by use of alloys and anticorrosion agents, the development of a method whereby titanium could be quickly and readily provided with a substantially impervious coating would be very desirable.
It is known that the platinum-group metals are highly resistant to corrosion by most acids, with the relative corrosion resistance of these noble metals being Rh.apprxeq.Ir&gt;Pt&gt;Pd&gt;Ru.apprxeq.Os. Corrosion, vol. 1, chapter 6 (L. L. Shreir, Ed. 1963).* Coatings of these metals have been used to protect substrate metals such as copper, brass, bronze, Ni, Ag, Au and Pt from corrosion. However, the effectiveness of such coatings depends not only on the ability of the coating metal to resist corrosion, but also on the avoidance of any galvanic corrosion between the base metal and the noble metal coating. Thus, rhodium is generally not plated directly over steel, zinc, aluminum, lead, tin and most tin-lead alloys; this group of base metals generally requires substantially non-porous undercoatings of copper, nickel or silver first be applied, since the inevitable development of pinholes in the rhodium coating, would, if the underlying metal were left exposed, be subject to corrosion of the base metal. Moreover, a known disadvantage of rhodium coatings is a high internal tensile stress which can give rise to cracking in deposits thicker than 0.1 mil. C. G. Fink, et al. Trans. Electrochem. Soc., vol. 63, pp. 181-186 (1933); E. H. Laister, et al., Trans. Inst. Metal Finishing, vol. 29, pp. 1-22 (1953); J. M. Hosdowitch Surface Protection Against Wear and Corrosion, pp. 52-55 (Amer. Soc. for Metals 1954); E. A. Parker, Plating, vol. 42, pp. 882-892 (1955); F. H. Reed, Trans. Inst. Metal Finishing, vol. 36, pp. 74-81 (1959); R. R. Benham, Platinum Metals Rev. 5(1), pp. 13-18 (1961); R. H. Atkinson, Modern Electroplating, pp. 310-325 (John Wiley & Sons, Inc. 1963); Corrosion, vol. 2, pp. 14.100-14.103, (L. L. Shreir, Ed. 1963). Exemplary of processes for providing the rhodium coatings of the prior art are the brushplating process disclosed in C. D. Hughes, Trans. Inst. Met. Finishing, vol. 33, pp. 424,439 (1956) and the electrodeposition processes of U.S. Pat. Nos. 1,949,131 and 1,981,820. FNT * See alsoM. Stern et al., supra at p. 760.
Dimensionally stable titanium anodes having coatings containing rhodium have been prepared by methods which require the anodes to be heated to a high temperature which acts to either oxidize the Rh-surface to form a protective layer over the Ti or to cause the Rh to diffuse into the surface of the Ti substrate metal, in effect forming a Rh--Ti alloy at the surface. However, the requirement of heat treatment using such high temperatures imposes considerable economic penalties, especially on one who seeks to protect existing chemical process apparatus or to manufacture large scale titanium equipment such as distillation towers and the like. As to the preparation of such anodes, see German Pat. No. 2,200,527, as cited in 77 Chem. Abs. 134,374 (1972) (Rh--Ru alloy layer); German Pat. Nos. 2,136,391 and 2,136,394, as cited in 76 Chem. Abs. 93908j and 93909k (1972), respectively (Rh--W and Rh--Te complex oxides); German Pat. No. 2,163,257, as cited in 77 Chem. Abs. 108,851g (1972) (sequential Rh and Rh--Ru layers); German Pat. No. 2,233,485, as cited in 78 Chem. Abs. 105,435j (1973) (complex Rh--Sb/Nb/Ta--Ru/Ir oxides); U.S. Pat. No. 3,801,490, as cited in 80 Chem. Abs. 152,139s (1974 (Bi--Rh oxides); O. Suzuki et al., 80 Chem. Abs. 66,206e, 66,207f, 66,208g, 66,209h and 66,210b (1974) (Ru-noble metal alloy coatings); and German Pat. No. 2,331,959, as cited in 80 Chem. Abs. 103,267y (1974) (mixed Ru, Ir, Rh and Pd oxides).
In the absence of such heating steps Rh-coated Ti anodes exhibited unpredictable corrosion properties. For example, M. Antler et al., 5 Electrochem. Tech. 126-130 (1967), 66 Chem. Abs. 101070r (1967) tested rhodium coated Ti anodes in the electrolysis of chloride and chloride-chlorate solutions and found corrosion films to develop which were not self-limiting and which spread under the rhodium coatings, which were themselves found to have detectable porosity. Also, the uncoated parts of the Ti anodes were corroded. S. P. Antonov, et al., 78 Chem. Abs. 78,899e (1973) studied the effectiveness of Ti and other metals as anode substrates for depositing thin-layer coatings of Pd, Rh, Pt and PbO.sub.2, which were applied after degreasing and etching of the selected substrate, and observed increased corrosion resistance in H.sub.2 SO.sub.4 --Cr(SO.sub.4).sub.3 and H.sub.2 SO.sub.4 --ZnSO.sub.4 media.
However, the foregoing methods employed for preparation of titanium anodes are not readily adaptable to preparation of titanium substrates which are intended for use in non-electrolytic environments. The art has heretofore required titanium articles which are coated with rhodium to be prepared via methods which employ a high temperature heating step. Thus, in Japanese Patent Publication 71/12,882, as cited in 76 Chem. Abs. 89,375r (1972) titanium articles were etched, dipped into noble metal salt solutions and then plated by heating of the surface at 600.degree. C. for one hour. Similarly, Japanese Kokai No. 73/25,636, after activating the titanium surface, dipped the titanium article into a solution containing the selected rhodium salt and then heated the treated article to a temperature above the decomposition temperature of the precious metal salt to effect diffusion of the noble metal into the surface of the titanium and thus created a rhodium-titanium alloy on the surface of the metal. Japanese Kokai No. 78/26,234, as cited in 89 Chem. Abs. 82,148d (1978) required heating of the rhodium plated article at a temperature of 600.degree. C. in air to form a rhodium oxide layer. Again, these methods are severely uneconomic and impracticable for protecting existing titanium equipment and are also only with great difficulty in fabrication of large, industrially-used chemical process apparatus. Moreover, Japanese Kokai No. 73/25,636 suggests, even though no working example to rhodium-coatings is presented, that corrosion will result even if such a heating step is used, since the Kokai's examples showed that a Pd coated Ti article, when exposed to a 5% boiling HCl solution, corroded at the rate of 0.32 mm/year, i.e., 12.6 mil/year, after only 8 hours of exposure.